Clocking Neptune’s mysterious spin

A day on Neptune lasts 15 hours, 57 minutes and 59 seconds, according to the first accurate measurement of its rotational period. University of Arizona (UA) planetary scientist Erich Karkoschka’s revealing study of Neptune’s rotation is a feat unachieved for any gas planet in our solar system except for Jupiter. It is one of the largest improvements in determining the rotational period of a gas planet in almost 350 years — ever since Italian astronomer Giovanni Cassini made the first observations of Jupiter’s Red Spot. Karkoschka’s study was accepted in May 2011 for publication in Icarus.

In this video, contrasting colors emphasize the planet’s atmospheric features — normally a pale blue.

Karkoschka, a senior staff scientist at UA’s Lunar and Planetary Laboratory, said:

The rotational period of a planet is one of its fundamental properties. Neptune has two features observable with the Hubble Space Telescope that seem to track the interior rotation of the planet. Nothing similar has been seen before on any of the four giant planets.

Combined color and near-infrared image of Neptune by the Hubble Space Telescope. It shows bands of methane in its atmosphere, along with its four moons, Proteus (outer brightest), Larissa, Galatea, and Despina. Image Credit: NASA/ESA

Unlike the rocky planets –- Mercury, Venus, Earth and Mars –- which behave like solid balls spinning in a rather straightforward manner, the giant gas planets –- Jupiter, Saturn, Uranus and Neptune — rotate more like giant blobs of liquid. Since they are believed to consist of mainly ice and gas around a relatively small solid core, their rotation involves a lot of sloshing, swirling and roiling, which has made it difficult for astronomers to get an accurate grip on exactly how fast they spin.

Karkoschka explained:

On Neptune, all you see is moving clouds and features in the planet’s atmosphere. Some move faster, some move slower, some accelerate, but you really don’t know what the rotational period is, if there even is some solid inner core that is rotating.

Karkoschka said Voyagers 1 and 2 found radio signals for Neptune, and astronomers thought they knew the rotation period of the planet. But when the Cassini probe made startling discoveries about Saturn’s northern and southern hemispheres rotating at different speeds, astronomers began to get a clearer picture of how the magnetic field behaves. Karkoschka explained:

The interior is rotating and drags the magnetic field along, but because of the solar wind or other unknown influences, the magnetic field cannot keep up with respect to the planet’s core and lags behind.

Karkoschka took advantage of what one might call the scraps of space science: publicly available images of Neptune from the Hubble Space Telescope archive. With unwavering determination, he then pored over hundreds of images, recording every detail and tracking distinctive features over long periods of time.

When I looked at the images, I found Neptune’s rotation to be faster than what Voyager observed. I think the accuracy of my data is about 1,000 times better than what we had based on the Voyager measurements –- a huge improvement in determining the exact rotational period of Neptune, which hasn’t happened for any of the giant planets for the last three centuries.

True color of Neptune as seen by the Voyager 2 spacecraft in 1989. Image Credit: NASA

Karkoschka discovered that two features in Neptune’s atmosphere stand out because they rotate steadily. Named the South Polar Feature and the South Polar Wave, the features are likely vortices swirling in the atmosphere, similar to Jupiter’s famous Red Spot, which can last for a long time. Karkoschka was able to track them over the course of more than 20 years. An observer watching the planet turn from a fixed spot in space would see both features appear every 15.9663 hours, with less than a few seconds of variation. Karkoschka said:

The regularity suggests those features are connected to Neptune’s interior in some way. How they are connected is up to speculation.

I thought the extraordinary regularity of Neptune’s rotation indicated by the two features was something really special.

Karkoschka discovered six more features that rotate with the same speed. In addition to getting a better grip on Neptune’s rotational period, the study could lead to a better understanding of the giant gas planets in general.

Summary: Erich Karkoschka, University of Arizona, has produced the first accurate measurement of Neptune’s rotational period. Results of his study were accepted in May 2011 for publication in Icarus.

Read more at University of Arizona News

Sloane Wiktorowicz and Andrew Ingersoll on future Neptune seas

July 3, 2011

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